1. Field of the Invention
The present invention relates to an electrostatic image developing toner that makes visible an electrostatic latent image formed by electrophotography, electrostatic printing or electrostatic recording, a developer using the toner, an image forming apparatus, and an image forming method.
2. Description of the Related Art
Of the above-mentioned printing or recording methods, for example, the electrophotography involves charging and exposing the photoconductor, creating an electrostatic latent image on the photoconductor, developing the electrostatic latent image with a particulate toner containing a coloring agent having a binder of resin, transferring and fixing the obtained toner image on the recording paper to produce a recorded image.
In such an electrostatic image recording process, the step of developing the electrostatic latent image with the particulate toner and the step of fixing the electrostatic latent image on the recording paper are particularly important. Conventionally, the typical method for developing the toner is the magnetic brush developing method using a two-component system composed of the toner and a magnetic carrier capable of high speed, high image quality development. Also, the method for fixing the toner is typically the heat roller fixing method with high heat efficiency and capable of high speed fixing.
On the other hand, recently, along with the development of information equipment, a laser beam printer using a laser beam for exposure of the photoconductor has been put commercialized in which a recording image is reproduced in dots by a modulation signal upon an instruction from the computer. Particularly in the recent laser beam printers, there is a demand for forming the image at higher quality, whereby the diameter of laser beam is restricted and reduced to have a high dot density of 600 to 1200 dpi (dots/inch). For the purpose of developing the finer electrostatic latent image, the particle size of toner and carrier is smaller, whereby the particulate toner has a volume average particle size of 10 μm or less and the particulate carrier has a weight average particle size of 100 μm or less. These particulate toner and particulate carrier are progressively employed.
On the other hand, the heat roller is often used for fixing. From the viewpoints of suppressing overheat deterioration of the printer and preventing heat deterioration of the cabin parts, shortening the warm-up time for activating the fixing unit to be ready for fixing, preventing a fixing failure due to heat absorbed into the recording sheet to keep the image quality through the continuous paper feed, preventing the recording sheet from being curled or burnt due to over heating, and reducing the load on the heat roller to simplify the structure of the fixing unit and reduce the size, there is a demand for developing the toner that can be fixed with lower power consumption of a fixing roller and a drive motor, using the heat roller at the lower temperatures and lower pressures.
Thus, there is a demand for developing the high performance particulate toner that can be fixed at lower temperatures and lower pressures. On the other hand, when the toner has particle size of 10 μm or less, as previously described, there are following problems. That is, in the developing process, though the high image quality is attained by using the particulate toner, the toner adherence (fogging) to non-image parts and the toner splash are likely to occur, possibly impairing the handling of the toner such as toner conveyance due to lower fluidity.
Moreover, due to strong adherence and weak shock resistance of the particulate toner, the carrier contamination (carrier spent) by the toner is likely to occur, possibly lowering the developer life. In fixing, the particulate toner requires more energy than the toner having large particle size to attain the same fixing strength, and there is a lower yield in the powdering and classification process when manufacturing the toner, so that the cost of the toner is raised.
The particulate toner has many problems as described above. Usually, it is difficult to practically use the toner having an average particle size of less than 3 μm. It is common to classify the average particle size of the toner in a range of from 3 to 10 μm, and increase the fluidity with the improved outside additives or outside treatment for the toner. On the other hand, the carrier has a small particle size of 100 μm or less, the specific surface area of the carrier being increased to improve the frictional electrification with the toner, when the particle size of the toner is smaller. However, the carrier of less than 30 μm has a lower magnetic force, and is more likely to adhere onto an electrostatic image holding member due to an electrostatic suction force. Therefore, the average particle size of the carrier is classified in a range of from 30 to 100 μm. If required, the surface of carrier is coated with resin.
With the better particle grading distribution and the improved fluidity and charging ability, the particulate toner and the developer have been put to practical use on the copying machine and the printer. However, when the printing is performed by the actual machine, especially when the high speed printing at 10 pages or more per minute is repeated, there is a problem peculiar to the particulate toner, in which the developer life is shortened by a carrier spent with the toner, and the photoconductor life is shortened by a photoconductor filming with the toner. The fixing strength of image is difficult to attain, and it is necessary to increase the temperature and pressure of the heat roller, especially in the fixing process. Thus, there was a problem that it was difficult for the fixing unit to achieve higher reliability, simplification and miniaturization, and cost reduction.
To enhance the fixing performance of the toner, it is well known to add a wax to the fixing resin. For example, several techniques were disclosed in JP-B-52-3304, JP-B-52-3305 And JP-B-57-52574.
Waxes are employed to prevent the toner from adhering to the heat roller when the toner has lower temperatures or higher temperatures, namely, a so-called offset phenomenon, and enhance the fixing property of the toner at lower temperatures. Recently, a low melting point wax has gained attention from the viewpoint of low temperature fixing.
For example, in JP-A-5-313413, it was disclosed that ethylene or propylene having a viscosity of 10000 poise or less at 140° C. and α-olefin copolymer were added to vinyl copolymer having a specific molecular weight distribution to improve the toner in terms of the fixing property at lower temperatures, offset resistance, and non-condensable property.
For the same purpose, JP-A-7-287413 disclosed that the paraffin wax having an absorbed heat quantity peak (melting point) at 75 to 85° C. measured by the differential scanning calorimeter (DSC) was added. Further, JP-A-8-314181, JP-A-9-179335 and JP-A-9-319139 disclosed that the natural gas Fischer-Tropsch wax having a melting point of 85 to 100° C. measured by the DSC was added, JP-A-6-324513 disclosed that polyethylene wax having a melting point of from 85 to 110° C. measured by the DSC was added, JP-A-7-36218 disclosed that polyethylene wax having a melting point of from 70 to 120° C. measured by the DSC from which components having a melting point of 50° C. or less were removed by distillation was added, and JP-A-8-114942 disclosed that polyethylene wax having a weight average molecular weight (Mw) of less than 1000 was added.
On the other hand, if a low melting point wax is added to the toner, the toner is degraded in the heat resistance, durability, preservation stability and fluidity. To improve them, JP-A-6-123994 disclosed that the wax having a weight average molecular weight (Mw)/a number average molecular weight (Mn) of 1.5 or less was used, JP-A-7-209909 disclosed that ethylene olefin copolymer wax having a melt viscosity of 0.5 to 10 mPa·s at 140° C., and a penetration of 3.0 dmm or less was used, and JP-A-7-287418 disclosed that Fischer-Tropsch wax having an average molecular weight of 1000 or more was used.
It is possible to enhance the fixing performance of toner by using these conventional techniques. However, when the low melting point wax is used, it is difficult to enhance the fixing performance of toner while keeping the heat resistance, durability, preservation stability and fluidity for the toner, especially with the particulate toner. Thus, the toner and the image forming method capable of being practically employed could not be provided.